Flash memory device and a fabrication process thereof, method of forming a dielectric film

ABSTRACT

A fabrication process of a flash memory device includes microwave excitation of high-density plasma in a mixed gas of Kr and an oxidizing gas or a nitriding gas. The resultant atomic state oxygen O* or hydrogen nitride radicals NH* are used for nitridation or oxidation of a polysilicon electrode surface. It is also disclosed the method of forming an oxide film and a nitride film on a polysilicon film according to such a plasma processing.

This is divisional application of U.S. application Ser. No. 10/359,714,filed on Feb. 7, 2003, which is a divisional of application Ser. No.09/867,699, filed on May 31, 2001, now U.S. Pat. 6,551,948, which is acontinuation application of PCT/JP01/01967, filed on Mar. 13, 2001,designating the U.S., the contents of all of which are incorporated intheir entirety by reference.

TECHNICAL FIELD

The present invention generally relates to semiconductor devices and afabrication process thereof. More particularly, the present inventionrelates to a method of forming a dielectric film and fabrication processof a non-volatile semiconductor memory device capable of rewritinginformation electrically, including a flash memory device.

There are various volatile memory devices such as DRAMs and SRAMs.Further, there are non-volatile memory devices such as a mask ROM, PROM,EPROM, EEPROM, and the like. Particularly, a flash memory device is anEEPROM having a single transistor for one memory cell and has anadvantageous feature of small cell size, large storage capacity and lowpower consumption. Thus, intensive efforts are being made on theimprovement of flash memory devices. In order that a flash memory devicecan be used stably over a long interval of time with low voltage, it isessential to use a uniform insulation film having high film quality.

BACKGROUND ART

First, the construction of a conventional flash memory device will beexplained with reference to FIG. 1 showing the concept of a generallyused flash memory device having a so-called stacked gate structure.

Referring to FIG. 1, the flash memory device is constructed on a siliconsubstrate 1700 and includes a source region 1701 and a drain region 1702formed in the silicon substrate 1700, a tunneling gate oxide film 1703formed on the silicon substrate 1700 between the source region 1701 andthe drain region 1702, and a floating gate 1704 formed on the tunnelinggate oxide film 1703, wherein there is formed a consecutive stacking ofa silicon oxide film 1705, a silicon nitride film 1706 and a siliconoxide film 1707 on the floating gate 1704, and a control gate 1708 isformed further on the silicon oxide film 1707. Thus, the flash memory ofsuch a stacked structure includes a stacked structure in which thefloating gate 1704 and the control gate 1708 sandwich an insulatingstructure formed of the insulation films 1705, 1706 and 1707therebetween.

The insulating structure provided between the floating gate 1704 and thecontrol gate 1705 is generally formed to have a so-called ONO structurein which the nitride film 1706 is sandwiched by the oxide films 1705 and1707 for suppressing the leakage current between the floating gate 1704and the control gate 1705. In an ordinary flash memory device, thetunneling gate oxide film 1703 and the silicon oxide film 1705 areformed by a thermal oxidation process, while the silicon nitride film1706 and the silicon oxide film 1707 are formed by a CVD process. Thesilicon oxide film 1705 may be formed by a CVD process. The tunnelinggate oxide film 1703 has a thickness of about 8 nm, while the insulationfilms 1705, 1706 and 1707 are formed to have a total thickness of about15 nm in terms of oxide equivalent thickness. Further, a low-voltagetransistor having a gate oxide film of 3-7 nm in thickness and ahigh-voltage transistor having a gate oxide film of 15-30 nm inthickness are formed on the same silicon in addition to the foregoingmemory cell.

In the flash memory cell having such a stacked structure, a voltage ofabout 5-7V is applied for example to the drain 1702 when writinginformation together with a high voltage larger than 12V applied to thecontrol gate 1708. By doing so, the channel hot electrons formed in thevicinity of the drain region 1702 are accumulated in the floating gatevia the tunneling insulation film 1703. When erasing the electrons thusaccumulated, the drain region 1702 is made floating and the control gate1708 is grounded. Further, a high voltage larger than 12V is applied tothe source region 1701 for pulling out the electrons accumulated in thefloating gate 1704 to the source region 1701.

Such a conventional flash memory device, on the other hand, requires ahigh voltage at the time of writing or erasing of information, while theuse of such a high voltage tends to cause a large substrate current. Thelarge substrate current, in turn, causes the problem of deterioration ofthe tunneling insulation film and hence the degradation of deviceperformance. Further, the use of such a high voltage limits the numberof times rewriting of information can be made in a flash memory deviceand also causes the problem of erroneous erasing.

The reason a high voltage has been needed in conventional flash memorydevices is that the ONO film, formed of the insulation films 1705, 1706and 1707, has a large thickness.

In the conventional art of film formation, there has been a problem,when a high-temperature process such as thermal oxidation process isused in the process of forming an oxide film such as the insulation film1705 on the floating gate 1704, in that the quality of the interfacebetween the polysilicon gate 1704 and the oxide film tends to becomepoor due to the thermal budget effect, etc. In order to avoid thisproblem, one may use a low temperature process such as CVD process forforming the oxide film. However, it has been difficult to form ahigh-quality oxide film according to such a low-temperature process.Because of this reason, conventional flash memory devices had to use alarge thickness for the insulation films 1705, 1706 and 1707 so as tosuppress the leakage current.

However, the use of large thickness for the insulation films 1705, 1706and 1707 in these conventional flash memory devices has caused theproblem in that it is necessary to use a large writing voltage and alsoa large erasing voltage. As a result of using large writing voltage andlarge erasing voltage, it has been necessary to form the tunneling gateinsulation film 1703 with large thickness so as to endure the largevoltage used.

DISCLOSURE OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful flash memory device and fabrication process thereofand further a method of forming an insulation film, wherein theforegoing problems are eliminated.

Another and more specific object of the present invention is to providea high-performance flash memory device having a high-quality insulationfilm that is formed at a low temperature process, the thickness of thetunneling gate insulation film or the thickness of the insulation filmbetween the floating gate and the control gate can be reducedsuccessfully without causing the problem of leakage current, andenabling writing and erasing at low voltage.

Another object of the present invention is to provide a method offorming an insulation film wherein a high-quality insulation film can beformed on polysilicon.

Another object of the present invention is to provide a flash memorydevice, comprising:

-   -   a silicon substrate,    -   a first electrode formed on the silicon substrate with a        tunneling insulation film interposed therebetween, and    -   a second electrode formed on the first electrode with an        insulation film interposed therebetween,    -   said insulation film having a stacked structure including at        least one silicon oxide film and one silicon nitride film, at        least a part of said silicon oxide film containing Kr with a        surface density of 10¹⁰cm⁻² or more.

According to the present invention, the quality of the insulation filmused in a flash memory device between a floating gate electrode and acontrol gate electrode is improved by forming the insulation film by anoxidation reaction or nitriding reaction conducted in Ar or Kr plasma inwhich atomic state oxygen O* or hydrogen nitride radicals NH* are formedefficiently. Further, it becomes possible to reduce the thickness of theinsulation film without causing unwanted increase of leakage current. Asa result, the flash memory device of the present invention can operateat high speed with low voltage and has a long lifetime.

Another object of the present invention is to provide a method offabricating a flash memory device comprising a silicon substrate, afirst electrode of polysilicon formed on the silicon substrate with aninsulation film interposed therebetween, and a second electrode formedon the first electrode with an inter-electrode insulation filminterposed therebetween, said inter-electrode insulation film having astacked structure containing at least one silicon oxide film and onesilicon nitride film,

-   -   said silicon oxide film being formed by the step of exposing a        silicon oxide film deposited by a CVD process to atomic state        oxygen O* formed by microwave excitation of plasma in a mixed        gas of an oxygen-containing gas and an inert gas predominantly        of a Kr gas.

According to the present invention, an oxide film having excellentleakage current characteristic is obtained for the inter-electrodeinsulation film, and it becomes possible to form a flash memory having asimple structure, capable of holding electric charges in the floatinggate electrode stably, and is operable at a low driving voltage.

Another object of the present invention is to provide a fabricationprocess of a flash memory device comprising a silicon substrate, a firstelectrode of polysilicon formed on the silicon substrate with aninsulation film interposed therebetween, and a second electrode formedon the first electrode with an inter-electrode insulation filminterposed therebetween, said inter-electrode insulation film having astacked structure including at least one silicon oxide film and onesilicon nitride film,

-   -   said silicon nitride film being formed by exposing a silicon        nitride film deposited by a CVD process to hydrogen nitride        radicals NH* formed by microwave excitation of plasma in a mixed        gas of an NH₃ gas or alternatively a gas containing N₂ and H₂        and a gas predominantly formed of an Ar or Kr gas.

According to the present invention, a nitride film having excellentleakage current characteristic suitable for the inter-electrodeinsulation film is obtained. Thus, it becomes possible to realize aflash memory having a simple construction and is capable of holdingelectric charges stably in the floating gate electrode. The flash memorythus obtained is operable at a low driving voltage.

Another object of the present invention is to provide a method offorming a silicon oxide film, comprising the steps of:

-   -   depositing a polysilicon film on a substrate; and    -   forming a silicon oxide film on a surface of said polysilicon        film by exposing the surface of said polysilicon film to atomic        state oxygen O* formed by microwave excitation of plasma in a        mixed gas of a gas containing oxygen and an inert gas        predominantly of a Kr gas.

According to the present invention, it becomes possible to form ahomogeneous silicon oxide film on a polysilicon film with uniformthickness irrespective of the orientation of the silicon crystalstherein. The silicon oxide film thus formed has excellent leakagecurrent characteristic comparative to that of a thermal oxide film andcauses a Fowler-Nordheim tunneling similarly to the case of a thermaloxide film.

Another object of the present invention is to provide a method offorming a silicon nitride film, comprising the steps of:

-   -   depositing a polysilicon film on a substrate; and    -   forming a nitride film on a surface of said polysilicon film by        exposing the surface of said polysilicon film to hydrogen        nitride radicals NH* formed by microwave excitation of plasma in        a mixed gas of a gas containing nitrogen and hydrogen as        constituent elements and an inert gas predominantly of an Ar gas        or a Kr gas.

According to the present invention, it becomes possible to form anitride film of excellent characteristic on the surface of a polysiliconfilm.

Another object of the present invention is to provide a method offorming a dielectric film, comprising the steps of:

-   -   depositing a polysilicon film on a substrate; and    -   converting a surface of said polysilicon film into a dielectric        film by exposing said polysilicon film to a microwave-excited        plasma formed in a mixed gas of an inert gas predominantly of Ar        or Kr and a gas containing oxygen as a constituent element and a        gas containing nitrogen as a constituent element.

According to the present invention, it becomes possible to form anoxynitride film having excellent characteristic on the surface of apolysilicon film.

Another object of the present invention is to provide a method offabricating a flash memory having a silicon substrate, a first electrodeof polysilicon formed on said silicon substrate with an insulation filminterposed therebetween, and a second electrode formed on said firstelectrode with an inter-electrode oxide film interposed therebetween,said inter-electrode oxide film being formed by the steps of:

-   -   depositing a polysilicon film on said silicon substrate as said        first electrode; and    -   exposing a surface of said polysilicon film to atomic state        oxygen O* formed by microwave excitation of plasma in a mixed        gas of a gas containing oxygen and an inert gas predominantly of        a Kr gas.

According to the present invention, an oxide film having excellentleakage current characteristic is obtained for the inter-electrodeinsulation film, and it becomes possible to realize a flash memoryhaving a simple construction and is capable of holding electric chargesin the floating gate electrode stably. The flash memory thus formed isoperable at a low driving voltage.

Another object of the present invention is to provide a method offabricating a flash memory having a silicon substrate, a first electrodeof polysilicon formed on said silicon substrate with an oxide filminterposed therebetween, and a second electrode of polysilicon formed onsaid first electrode with an inter-electrode nitride film interposedtherebetween, said inter-electrode nitride film being formed by thesteps of:

-   -   depositing a polysilicon film on said silicon substrate as said        first electrode; and    -   exposing a surface of said polysilicon film to hydrogen nitride        radicals NH* formed by microwave excitation of plasma in a mixed        gas of a gas containing nitrogen and hydrogen and an inert gas        predominantly of an Ar gas or a Kr gas.

According to the present invention, a nitride film having excellentleakage current characteristic is obtained for the inter-electrodenitride film and it becomes possible to realize a flash memory having asimple construction and is capable of holding electric charges in thefloating gate electrode stably. The flash memory thus formed is operableat a low driving voltage.

Another object of the present invention is to provide a method offabricating a flash memory having a silicon substrate, a first electrodeof polysilicon formed on said silicon substrate with insulation filminterposed therebetween, and a second electrode of polysilicon formed onsaid first electrode with an inter-electrode oxynitride film interposedtherebetween, said inter-electrode oxynitride film being formed by thesteps of:

-   -   depositing a polysilicon film on said silicon substrate as said        first electrode; and    -   converting a surface of said polysilicon film into a silicon        oxynitride film by exposing said polysilicon film to microwave        excited plasma formed in a mixed gas of an inert gas        predominantly of Ar or Kr and a gas containing oxygen and        nitrogen.

According to the present invention, an oxynitride film having excellentleakage current characteristic is obtained for the inter-electrodeinsulation film, and it becomes possible to realize a flash memorycapable of holding electric charges stably in the floating gateelectrode. The flash memory thus formed is operable at a low drivingvoltage.

Another object of the present invention is to provide a method offorming a silicon oxide film on a polysilicon film, comprising the stepsof:

-   -   forming atomic state oxygen O* in a processing vessel of a        microwave processing apparatus, said microwave processing        apparatus including: a shower plate in a part of said processing        vessel such that said shower plate extends parallel to a        substrate to be processed, said shower plate having a plurality        of apertures for supplying a plasma gas toward said substrate;        and a microwave radiation antenna emitting a microwave into said        processing vessel via said shower plate, by supplying a gas        predominantly of Kr and a gas containing oxygen into said        processing vessel via said shower plate and further by supplying        said microwave into said processing vessel from said microwave        radiation antenna through said shower plate; and    -   forming a silicon oxide film by causing oxidation in a surface        of a polysilicon film formed on said substrate by said plasma in        said processing vessel.

According to the present invention, atomic state oxygen that causeoxidation in a polysilicon film are formed efficiently by inducinghigh-density plasma of low electron temperature in the processingchamber as a result of microwave excitation of the plasma gas supplieduniformly from the shower plate. The silicon oxide film thus formed bythe Kr plasma is irrelevant to the crystal orientation of the Sicrystals on which the silicon oxide film is formed. Thus, the siliconoxide film is formed uniformly on the polysilicon film. The siliconoxide film contains little surface states and is characterized by smallleakage current. According to the present invention, the oxidationprocessing of the polysilicon film can be conducted at a low temperatureof 550° C. or less, and there occurs no substantial grain growth in thepolysilicon film even when such an oxidation process is conducted. Thus,the problem of concentration of electric field, and the like, whicharises with such a grain growth is avoided.

Another object of the present invention is to provide a method offorming a silicon nitride film on a polysilicon film, said methodcomprising the steps of:

-   -   forming plasma containing hydrogen nitride radicals NH* in a        processing vessel of a microwave processing apparatus, said        microwave processing apparatus including: a shower plate in a        part of said processing vessel so as to extend parallel to a        substrate to be processed, said shower plate having a plurality        of apertures for supplying a plasma gas to said substrate; and a        microwave radiation antenna emitting a microwave into said        processing vessel via said shower plate, by supplying a gas        predominantly of Ar or Kr and a gas containing nitrogen and        hydrogen into said processing vessel from said shower plate and        by further supplying said microwave into said processing vessel        from said microwave radiation antenna through said shower plate;        and    -   forming a silicon nitride film by nitriding a surface of a        polysilicon film formed on said substrate by said plasma in said        processing vessel.

According to the present invention, hydrogen nitride radicals NH* thatcause nitridation in the polysilicon film are formed efficiently byinducing high-density plasma having a low electron temperature in theprocessing chamber by microwave excitation of the plasma gas supplieduniformly from the shower plate. The silicon nitride film thus formed bythe Kr plasma has an advantageous feature of small leakage current inspite of the fact that the silicon nitride film is formed at a lowtemperature.

Another object of the present invention is to provide a method offabricating a flash memory device, said flash memory device having asilicon substrate and including a first electrode formed on said siliconsubstrate with a tunneling insulation film interposed therebetween and asecond electrode formed on said first electrode with an insulation filminterposed therebetween, said insulation film having a stacked structurecontaining at least one silicon oxide film and one silicon nitride film,said silicon oxide film being formed by the steps of:

-   -   introducing a gas containing oxygen and a gas predominantly of a        Kr gas into a processing chamber, and causing microwave        excitation of plasma in said processing chamber.

According to the present invention, it becomes possible to oxidize thesurface of the first electrode at low temperature, by conducting theoxidation processing in the Kr plasma in which atomic state oxygen O*are formed efficiently. As a result, an oxide film containing smallsurface states and is characterized by small leakage current can beobtained for the desired silicon oxide film.

Another object of the present invention is to provide a fabricationprocess of a flash memory device having a silicon substrate, a firstelectrode formed on said silicon substrate with a tunneling insulationfilm interposed therebetween, and a second electrode formed on saidfirst electrode with an insulation film interposed therebetween, saidinsulation film having a stacked structure containing at least onesilicon oxide film and one silicon nitride film,

-   -   said silicon nitride film being formed by introducing an NH₃ gas        or a gas containing N₂ and H₂ and a gas predominantly of Ar or        Kr into a processing chamber, and causing microwave excitation        of plasma in said processing chamber.

According to the present invention, it becomes possible to nitride thesurface of the first electrode at low temperature by conducting thenitridation in the plasma of Ar or Kr in which the hydrogen nitrideradicals NH* are formed efficiently.

Other objects and further features of the present invention will becomeapparent from the following detailed description of the invention whenread in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram showing a schematic cross-section ofa conventional flash memory device;

FIG. 2 is a diagram showing the concept of the plasma apparatus thatuses a radial line slot antenna:

FIG. 3 is a diagram showing the relationship between a thickness and agas pressure in a processing chamber for an oxide film formed accordingto a first embodiment of the present invention;

FIG. 4 is a diagram showing the relationship between the thickness andduration of oxidation for the oxide film formed according to the firstembodiment of the present invention;

FIG. 5 is a diagram showing the depth profile of Kr density in thesilicon oxide film according to the first embodiment of the presentinvention;

FIG. 6 is a diagram showing the surface state density in the siliconoxide film according to the first embodiment of the present invention;

FIG. 7 is a diagram showing the relationship between the surface statedensity and the breakdown voltage for the silicon oxide film accordingto the first embodiment of the present invention;

FIGS. 8A and 8B are diagrams showing the relationship between thesurface state density and break down voltage of the silicon oxide filmobtained according to the first embodiment of the present invention andthe total pressure of the processing chamber;

FIG. 9 is a diagram showing the dependence of film thickness on thetotal pressure used in the processing chamber for a nitride film formedaccording to a second embodiment of the present invention;

FIG. 10 is a diagram showing the current-voltage characteristic of thesilicon nitride film according to the second embodiment of the presentinvention;

FIGS. 11A and 11B are diagrams showing the oxidation process, nitridingprocess and oxy-nitriding process of a polysilicon film according to athird embodiment of the present invention;

FIG. 12 is a diagram showing the dependence of film thickness on theoxidation duration for an oxide film obtained by an oxidation processingof a polysilicon film according to a third embodiment of the presentinvention;

FIGS. 13A-13C are diagrams showing a change of surface morphologyassociated with the oxidation process of a polysilicon film according tothe third embodiment of the present invention;

FIGS. 14A and 14B are diagrams showing a change of surface morphology ofa polysilicon film when subjected to a thermal oxidation process;

FIGS. 15A and 15B are diagrams showing the transmission electronmicroscope image of a polysilicon film formed according to the thirdembodiment of the present invention;

FIGS. 16-17 are diagrams showing the electric properties of the oxidefilm formed on a polysilicon according to the third embodiment of thepresent invention in comparison with a thermal oxide film;

FIG. 18 is a diagram showing the cross-sectional structure of a flashmemory device according to a fourth embodiment of the present invention;

FIG. 19 is a diagram showing the cross-sectional structure of a flashmemory device according to a fifth embodiment of the present invention;

FIGS. 20-23 are diagrams showing the fabrication process of a flashmemory device according to a fifth embodiment of the present invention;

FIG. 24 is a diagram showing the cross-sectional structure of the flashmemory device according to a sixth embodiment of the present invention;and

FIG. 25 is a diagram showing the cross-sectional structure of a flashmemory device according to a seventh embodiment of the presentinvention.

BEST MODE FOR IMPLEMENTING THE INVENTION

Hereinafter, embodiments of the present invention will be described.

First Embodiment

First, low temperature oxide film formation using plasma will bedescribed.

FIG. 2 is a cross sectional diagram showing the construction of anexemplary microwave plasma processing apparatus used in the presentinvention for realizing the oxidation process, wherein the microwaveplasma processing apparatus uses a radial line slot antenna (seeWO98/33362). The novel feature of the present embodiment is to use Kr asthe plasma excitation gas at the time of forming the oxide film.

Referring to FIG. 2, the microwave plasma processing apparatus includesa vacuum vessel (processing chamber) 101 accommodating therein a stage104 on which a substrate 103 to be processed is supported. Theprocessing chamber 101 is evacuated to a vacuum state, and a Kr gas andan O₂ gas are introduced from a shower plate 102 formed at a part of thewall of the processing chamber 101 such the pressure inside theprocessing chamber is set to about 1 Torr (about 133 Pa). Further, adisk-shaped substrate such as a silicon wafer is placed on the stage 104as the foregoing substrate 103. The stage 104 includes a heatingmechanism, and the temperature of the substrate 103 is set to about 400°C. It is preferable to set the temperature in the range of 200-550° C.As long as the temperature is set in this range, a similar result isobtained.

Next, a microwave of 2.45 GHz is supplied from an external microwavesource via a coaxial waveguide 105 connected thereto, wherein themicrowave thus supplied is radiated into the processing chamber 101 bythe radial line slot antenna 106 through a dielectric plate 107. As aresult, there is formed high-density plasma in the processing chamber101. As long as the frequency of the microwave is in the range of 900MHz or more but not exceeding 10 GHz, a similar result is obtained asdescribed below. In the illustrated example, the distance between theshower plate 102 and the substrate 103 is set to about 6 cm. Narrowerthe distance, faster the film forming process.

In the microwave plasma processing apparatus of FIG. 2, it becomespossible to realize a plasma density exceeding 1×10¹²cm⁻³ at the surfaceof the substrate 103. Further, the high-density plasma thus formed bymicrowave excitation has a low electron temperature, and a plasmapotential of 10 V or less is realized at the surface of the substrate103. Thus, the problem of the substrate 103 being damaged by the plasmais positively eliminated. Further, there occurs no problem ofcontamination of the substrate 103 because of the absence of plasmasputtering in the processing chamber 101. Because of the fact that theplasma processing is conducted in a narrow space between the showerplate 102 and the substrate 103, the product material of the reactionflows quickly in the lateral direction to a large volume spacesurrounding the stage 104 and is evacuated. Thereby, a very uniformprocessing is realized.

In the high-density plasma in which an Kr gas and an O₂ gas are mixed,Kr* at the intermediate excitation state cause collision with the O₂molecules and there occurs efficient formation of atomic state oxygenO*, and the atomic state oxygen O* thus formed cause oxidation of thesubstrate surface. It should be noted that oxidation of a siliconsurface has conventionally been conducted by using H₂O or O₂ moleculesat very high process temperature such as 800° C. or more. In the case ofusing atomic state oxygen, on the other hand, it becomes possible tocarry out the oxidation process at a low temperature of 550° C. or less.

In order to increase the chance of collision between K* and O₂, it ispreferable to increase the pressure in the processing chamber 101. Onthe other hand, the use of too high pressure in the processing chamberincreases the chance that O* causing collision with another O* andreturning to the O₂ molecule. Thus, there would exist an optimum gaspressure.

FIG. 3 shows the thickness of the oxide film for the case in which thetotal pressure inside the processing chamber 101 is changed whilemaintaining the Kr and oxygen pressure ratio such that the proportion ofKr is 97% and the proportion of oxygen is 3%. In the experiment of FIG.3, it should be noted that the silicon substrate was held at 400° C. andthe oxidation was conducted over the duration of 10 minutes.

Referring to FIG. 3, it can be seen that the thickness of the oxide filmbecomes maximum when the total gas pressure in the processing chamber101 is set to 1 Torr, indicating that the oxidation process becomesoptimum under this pressure or in the vicinity of this pressure.Further, it should be noted that this optimum pressure remains the samein the case the silicon substrate has the (100) oriented surface andalso in the case the silicon substrate has the (111) oriented surface.

FIG. 4 shows the relationship between the thickness of the oxide filmand the duration of the oxidation processing for the oxide film that isformed by oxidation of the silicon substrate surface using the Kr/O₂high-density plasma. In FIG. 4, the result for the case in which thesilicon substrate has the (100) oriented surface and the result for thecase in which the silicon substrate has the (111) oriented surface areboth represented. Further, FIG. 4 also represents the oxidation timedependence for the case a conventional dry oxidation process at thetemperature of 900° C. is employed.

Referring to FIG. 4, it can be seen that the oxidation rate caused bythe Kr/O₂ high-density plasma oxidation processing, conducted at thetemperature of 400° C. under the chamber pressure of 1 Torr, is largerthan the oxidation rate for a dry O₂ process conducted at 900° C. underthe atmospheric pressure.

In the case of conventional dry thermal oxidation process at 900° C. itcan be seen that the growth rate of the oxidation film is larger whenthe oxide film is formed on the (111) oriented silicon surface ascompared with the case of forming the oxide film on the (100) orientedsilicon surface. In the case in which the Kr/O₂ high-density plasmaoxidation process is used, on the other hand, this relationship isreversed and the growth rate of the oxide film on the (111) surface issmaller than the growth rate of the oxide film on the (100) surface. Inview of the fact that silicon atoms are arranged with larger surfacedensity on the (111) oriented surface than on the (100) oriented surfacein a Si substrate, it is predicted that the oxidation rate should besmaller on the (111) surface than on the (100) surface as long as thesupply rate of the oxygen radicals is the same. The result of theforegoing oxidation process of the silicon substrate surface is in goodconformity with this prediction when the Kr/O₂ high-density plasma isused for the oxidation process, indicating that there is formed a lodense oxide film similar to the one formed on a (100) surface, also onthe (111) surface. In the conventional case, on the other hand, theoxidation rate of the (111) surface is much larger than the oxidationrate of the (100) surface. This indicates that the oxide film formed onthe (111) film would be sparse in quality as compared with the oxidefilm formed on the (100) surface.

FIG. 5 shows the depth profile of the Kr density inside the siliconoxide film that is formed according to the foregoing process, whereinthe depth profile FIG. 5 was obtained by a total-reflection fluorescentX-ray spectrometer. In the experiment of FIG. 5, the formation of thesilicon oxide film was conducted at the substrate temperature of 400° C.while setting the oxygen partial pressure in the Kr gas to 3% andsetting the pressure of the processing chamber to 1 Torr (about 133 Pa).

Referring to FIG. 5, it can be seen that the surface density of Krdecreases toward the silicon/silicon oxide interface, and a density of2×10¹¹cm⁻² is observed at the surface of the silicon oxide film. Thus,the result of FIG. 5 indicates that a substantially uniform Krconcentration is realized in the silicon oxide film when the siliconoxide film is formed by surface oxidation of a silicon substrate whileusing the Kr/O₂ high-density plasma, provided that the silicon oxidefilm has a thickness of 4 nm or more. It can be seen that the Krconcentration in the silicon oxide film decreases toward thesilicon/silicon oxide surface. According to the method of silicon oxideformation of the present invention, Kr is incorporated in the siliconoxide film with a surface density of 10¹⁰cm⁻² or more. The result ofFIG. 5 is obtained on the (100) surface and also on the (111) surface.

FIG. 6 shows the surface state density formed in an oxide film, whereinthe result of FIG. 6 was obtained by a low-frequency C-V measurement.The silicon oxide film of FIG. 6 was formed at the substrate temperatureof 400° C. while using the apparatus of FIG. 2. In the experiment, theoxygen partial pressure in the rare gas was set to 3% and the pressurein the processing chamber was set to 1 Torr (about 133 Pa). For the sakeof comparison, the surface state density of a thermal oxide film formedat 900° C. in a 100% oxygen atmosphere is also represented.

Referring to FIG. 6, it can be seen that the surface state density ofthe oxide film is small in both of the cases in which the oxide film isformed on the (100) surface and in which the oxide film is formed on the(111) surface as long as the oxide film is formed while using the Krgas. The value of the surface state density thus achieved is comparablewith the surface state density of a thermal oxide film that is formed onthe (100) surface in a dry oxidation atmosphere at 900° C. Contrary tothe foregoing, the thermal oxide film formed on the (111) surface has asurface state density larger than the foregoing surface state density bya factor of 10.

The mechanism of the foregoing results is thought as follows.

Viewing the silicon crystal from the side of the silicon oxide film,there appear two bonds for one silicon atom when the silicon surface isthe (100) surface. On the other hand, there appear one bond and threebonds alternately for one silicon atom when the silicon surface is the(111) surface. Thus, when a conventional thermal oxidation process isapplied to a (111) surface, oxygen atoms quickly cause bonding to allthe foregoing three bonds, leaving the remaining bond behind the siliconatom. Thereby, the remaining bond may extend and form a weak bond ordisconnected and form a dangling bond. When this is the case, thereinevitably occurs an increase of surface state density.

When the high-density plasma oxidation is conducted in the mixed gas ofKr and O₂, Kr* of the intermediate excitation state cause collision withO₂ molecules and there occurs efficient formation of atomic state oxygenO*, wherein the atomic state oxygen O* thus formed easily reach the weakbond or dangling bond noted before and form a new silicon-oxygen bond.With this, it is believed that the surface states are reduced also onthe (111) surface.

In the experiment for measuring the relationship between the oxygenpartial pressure in the Kr gas used for the atmosphere during theformation of the silicon oxide film and the breakdown voltage of thesilicon insulation film thus formed, and further in the experiment formeasuring the relationship between the oxygen partial pressure in the Krgas and the surface state density in the silicon oxide film thus formed,it was confirmed that a generally same result is obtained for the casein which the silicon oxide film is formed on the (100) surface and forthe case in which the silicon oxide film is formed on the (111) surface,and that the surface state density becomes minimum when the oxygenpartial pressure in the Kr gas is set to 3%, provided that the siliconoxide film is formed by setting the pressure of the processing chamberto 1 Torr (about 133 Pa). Further, the breakdown voltage of the siliconoxide film becomes maximum when the oxygen partial pressure is set toabout 3%. From the foregoing, it is derived that an oxygen partialpressure of 2-4% is preferable for conducting the oxidation process byusing the Kr/O₂ mixed gas.

FIG. 7 shows a relationship between the pressure used for forming thesilicon oxide film and the breakdown voltage of the silicon oxide filmthus formed. Further, FIG. 7 shows the relationship between the pressureand the surface state density of the silicon oxide film. In FIG. 7, itshould be noted that the oxygen partial pressure is set to 3%.

Referring to FIG. 7, it can be seen that the breakdown voltage of thesilicon oxide film becomes maximum and the surface state density becomesminimum when the pressure of about 1 Torr is used at the time of formingthe oxide film. From the result of FIG. 7, it is concluded that thepreferable pressure of forming an oxide film by using a Kr/O₂ mixed gaswould be 800-1200 mTorr. The result of FIG. 7 is valid not only for theprocess on the (100) surface but also for the process on the (111)surface.

In addition to the foregoing, other various preferable characteristicswere obtained for the oxide film formed by the oxidation of siliconsubstrate surface by the Kr/O₂ high-density plasma with regard toelectronic and reliability characteristics, including the breakdowncharacteristic, the leakage characteristic, the hotcarrier resistance,and the QBD (Charge-to-Breakdown) characteristic, which represents theamount of electric charges that leads a silicon oxide film to breakdownas a result of application of a stress current, wherein thecharacteristics thus obtained are comparable to those of the thermaloxide film that is formed at 900° C.

FIGS. 8A and 8B show the leakage current induced by a stress current fora silicon oxide film thus obtained, in comparison with the case of aconventional thermal oxide film. In FIGS. 8A and 8B, the thermal oxidefilm has a thickness of 3.2 nm.

Referring to FIGS. 8A and 8B, it can be seen that there occurs anincrease of leakage current with injection of electric charges into theconventional thermal oxide film, while there occurs no such a change ofelectric current in the plasma oxide film that is formed by using theKr/O₂ plasma, even in the case electric charges of 100 C/cm² areinjected. Thus, the silicon oxide film of the present invention has avery long lifetime and it takes a very long time for a tunneling currentto cause degradation in the oxide film. The oxide film of the presentinvention is thus most suitable for the tunneling oxide film of a flashmemory device.

As noted previously, the oxide film grown by the Kr/O₂ high-densityplasma has a characteristic comparable with, or superior to, theconventional high-temperature thermal oxide film formed on the (100)surface, for both of cases in which the oxide film is grown on the (100)surface and the oxide film is grown on the (111) surface, in spite ofthe fact that the oxide film is formed at a low temperature of 400° C.It is noted that the existence of Kr in the oxide film contributes alsoto this effect. More specifically, the existence of Kr in the oxide filmcauses relaxation of stress at the Si/SiO₂ interface and decrease of theelectric charges in the film and the surface state density, leading toremarkable improvement of electric properties of the oxide film.Particularly, the existence of Kr atoms with a density of 10¹⁰cm⁻² asrepresented in FIG. 5 is believed to contribute to the improvement ofelectric properties and reliability properties of the silicon oxidefilm.

Second Embodiment

Next, the process of forming a nitride film at a low temperature byusing high-density microwave plasma will be described.

In the formation of the nitride film, the same apparatus as the oneexplained with reference to. FIG. 2 is used, except that Ar or Kr isused for the plasma excitation gas at the time of forming the nitridefilm.

Thus, the vacuum vessel (processing chamber) 101 is evacuated to a highvacuum state first, and the pressure inside the processing chamber 101is then set to about 100 mTorr (about 13 Pa) by introducing an Ar gasand a NH₃ gas via the shower plate 102, and the like. Further, adisk-shaped substrate such as a silicon wafer is placed on the stage 104as the substrate 103 and the substrate temperature is set to about 500°C. As long as the substrate temperature is in the range of 400-500° C.,almost the same results are obtained.

Next, a microwave of 2.45 GHz is introduced into the processing chamberfrom the coaxial waveguide 105 via the radial line slot antenna 106 andfurther through the dielectric plate 107, and there is inducedhigh-density plasma in the processing chamber. It should be noted that asimilar result is obtained as long as a microwave in the frequency of900 MHz or more but not exceeding 10 GHz is used. In the illustratedexample, the distance between the shower plate 102 and the substrate 103is set to 6 cm. Narrower the distance, faster the film formation rate.While the present embodiment shows the example of forming a film byusing the plasma apparatus that uses the radial line slot antenna, it ispossible to use other method for introducing the microwave into theprocessing chamber.

In the present embodiment, it should be noted that an Ar gas is used forexciting plasma. However, a similar result is obtained also when a Krgas is used. While the present embodiment uses NH₃ for the plasmaprocess gas, it is also possible to use a mixed gas of N₂ and H₂ forthis purpose.

In the high-density plasma excited in the mixed gas of Ar or Kr and NH₃(or alternatively N₂ and H₂), there are formed NH* radicals efficientlyby Ar* or Kr* having an intermediate excitation state, and the NH*radicals thus formed cause the desired nitridation of the substratesurface. Conventionally, there has been no report of direct nitridationof silicon surface. Thus, a nitride film has been formed by a plasma CVDprocess, and the like. However, the nitride film thus formed by aconventional plasma CVD process does not have the quality required for agate insulation film of a transistor. In the nitridation of siliconaccording to the present embodiment, on the other hand, it is possibleto form a high-quality nitride film at low temperature on any of the(100) surface and the (111) surface, irrespective of the surfaceorientation of the silicon substrate.

Meanwhile, it should be noted that existence of hydrogen is an importantfactor when forming a silicon nitride film. With the existence ofhydrogen in plasma, the dangling bonds existing in the silicon nitridefilm or at the nitride film interface are terminated in the form of Si—Hbond or N—H bond, and the problem of electron trapping within thesilicon nitride film or on the silicon nitride interface is eliminated.The existence of the Si—H bond and the N—H bond in the nitride film isconfirmed in the present invention by infrared absorption spectroscopyor X-ray photoelectron spectroscopy. As a result of the existence ofhydrogen, the hysteresis of the CV characteristic is also eliminated.Further, it is possible to suppress the surface state density of thesilicon/silicon nitride interface below 3×10¹⁰cm⁻² by setting thesubstrate temperature to 500° C. or more. In the event the siliconnitride film is formed by using an inert gas (Ar or Kr) and a mixed gasof N₂/H₂, the number of the traps of electrons or holes in the filmdecreases sharply by setting the partial pressure of the hydrogen gas to0.5% or more.

FIG. 9 shows the pressure dependence of the film thickness of thesilicon nitride film thus formed according to the foregoing process. Inthe illustrated example, the ratio of the Ar gas to the NH₃ gas is setto 98:2 in terms of partial pressure, and the film formation wasconducted over the duration of 30 minutes.

Referring to FIG. 9, it can be seen that the growth rate of the nitridefilm increases when the pressure in the processing chamber 101 isreduced so as to increase the energy given to NH₃ (or N₂(H₂) from theinert gas (Ar or Kr). From the viewpoint of efficiency of nitridation,it is therefore preferable to use the gas pressure of 50-100 mTorr(about 7-13 Pa). Further, it is preferable to set the partial pressureof NH₃ (or N₂/H₂) in the rare gas atmosphere to 1-10%, more preferablyto 2-6%.

It should be noted that the silicon nitride film of the presentembodiment has a dielectric constant of 7.9, which is almost twice aslarge as that of a silicon oxide film.

FIG. 10 shows the current-voltage characteristic of the silicon nitridefilm of the present embodiment. It should be noted that the result ofFIG. 10 is obtained for the case in which a silicon nitride film havinga thickness of 4.2 nm (2.1 nm in terms of oxide film equivalentthickness) is formed by using a gas mixture of Ar/N₂/H₂ while settingthe gas composition ratio, Ar:N₂:H₂, to 93:5:2 in terms of partialpressure. In FIG. 10, the result for the foregoing nitride film iscompared also with the case of a thermal oxide film having a thicknessof 2.1 nm.

Referring to FIG. 10, it can be seen that there is realized a very smallleakage current, smaller than the leakage current of a silicon oxidefilm by a factor of 10⁴ or more, is obtained when a voltage of 1V isapplied for the measurement. This result indicates that the siliconnitride film thus obtained can be used as the insulating film that isprovided between a floating gate electrode and a control gate electrodeof a flash memory device for suppressing the leakage current flowingtherebetween.

It should be noted that the foregoing condition of film formation, theproperty of the film, or the electric characteristic of the film areobtained similarly on any of the surfaces of the silicon crystal. Inother words, the same result is obtained on the (100) surface and alsoon the (111) surface. According to the present invention, therefore, itis possible to form a silicon nitride film of excellent quality on anyof the crystal surfaces of silicon. It should be noted that theexistence of the Si—H bond or N—H bond in the film is not the only causeof the foregoing advantageous effect of the present invention. Theexistence of Ar or Kr in the film contributes also to the foregoingadvantageous result. As a result of the existence of Ar or Kr in thefilm, it should be noted that the stress within the nitride film or thestress at the silicon/nitride film interface is relaxed substantially,while this relaxation of stress also contributes to the reduction offixed electric charges and the surface state density in the siliconnitride film, which leads to the remarkable improvement of electricproperties and reliability. Particularly, the existence of Ar or Kr withthe density of 10¹⁰cm⁻² is thought as contributing effectively to theimprovement of electric characteristics and reliability of the siliconnitride film, just in the case of the silicon oxide film represented inFIG. 5.

Thidr Embodiment

The foregoing method of forming oxide film or nitride film is applicablealso to the oxidation or nitridation of polysilicon. Thus, the presentinvention enables formation of a high-quality oxide film or nitride filmon polysilicon.

Hereinafter, the method of forming a dielectric film on a polysiliconfilm according to a third embodiment of the present invention will bedescribed with reference to FIGS. 11A and 11B.

Referring to FIG. 11A, a polysilicon film 203 is deposited on a siliconsubstrate 201 covered by an insulation film 202. By exposing thepolysilicon film 203 to the high-density mixed gas plasma of Kr or Arand oxygen in the processing vessel 101 of the microwave plasmaprocessing apparatus of FIG. 2 in the step of FIG. 11B, a silicon oxidefilm 204 having a high film quality is obtained on the surface of thepolysilicon film 203, wherein the silicon oxide film 204 thus formed ischaracterized by small surface state density and small leakage current.

In the step of FIG. 11B, it is also possible to form a high-qualitynitride film 205 on the surface of the polysilicon film 203 by exposingthe polysilicon film 203 to the high-density mixed gas plasma of Kr orAr and NH₃ or N₂ and H₂.

Further, it is possible, in the step of FIG. 11B, to form a high-qualityoxynitride film 206 on the surface of the polysilicon film 203, byexposing the polysilicon film 203 to the high-density mixed gas plasmaof Kr or Ar and oxygen and NH₃ or N₂ and H₂.

It should be noted that a polysilicon film formed on an insulation filmtends to take a stable state in which the (111) surface is oriented inthe direction perpendicular to the insulation film. The polysilicon filmhaving this state is dense and provides good quality. On the other hand,crystal grains of other crystal orientation may exist also in thepolysilicon film. According to the method of forming an oxide film or anitride film or an oxynitride film of the present embodiment, it becomespossible to form a high-quality oxide film, or a high-quality nitridefilm or a high-quality oxynitride film, irrespective of the surfaceorientation of silicon layer. Thus, the process of FIGS. 11A and 11B ismost suitable for forming a high quality thin oxide film or a nitridefilm or an oxynitride film on a polysilicon film. It should be notedthat the polysilicon film may be the first polysilicon gate electrodethat constitutes the floating electrode of flash memory. As the oxidefilm or nitride film or oxynitride film of the present invention can beformed at a low temperature of 550° C. or less, there arises no problemof rough surface formation on the polysilicon surface.

FIG. 12 shows the result of the experiment of forming an oxide film onan n-type polysilicon film having the thickness of 200 nm, wherein itshould be noted that the polysilicon film is formed on a thermal oxidefilm covering the (100) oriented surface of a Si substrate with athickness of 100 nm. It should be noted that FIG. 12 also shows the casein which the (100) surface and the (111) surface of a Si substrate isoxidized directly. In FIG. 12, the vertical axis represents thethickness of the oxide film thus formed, while the horizontal axisrepresents the duration of the process. Further, ▴ in FIG. 12 shows thecase in which an oxide film is formed by processing the polysiliconsurface thus formed by the Kr/O₂ plasma, while ● in FIG. 12 shows thecase in which an oxide film is formed by processing the (100) surface ofthe Si substrate by the Kr/O₂ plasma. Further, ▪ in FIG. 12 shows thecase in which an oxide film is formed by processing the (111) surface ofthe Si substrate by the Kr/O₂ plasma. In FIG. 12, it should further benoted that ∘ represents the case of causing thermal oxidation of the(100) surface of the Si substrate, while □ represents the case ofcausing a thermal oxidation of the (111) surface of the Si substrate.Further, Δ represents the case in which thermal oxidation is applied tothe surface of a polysilicon film. It should be noted that the Kr/O₂plasma processing was conducted at the temperature of 400° C., by usingthe apparatus explained already with reference to FIG. 2 while settingthe internal pressure of the processing chamber 101 to 1 Torr (about 133Pa) and setting the ratio of the Kr gas and the oxygen gas to 97:3 interms of flow-rate. On the other hand, the thermal oxidation process wasconducted at 900° C. in the 100% oxygen atmosphere. In the experiment ofFIG. 12, it should be noted that the polysilicon film is doped to acarrier density exceeding 10²⁰cm⁻³.

Referring to FIG. 12, no substantial difference of oxidation process canbe seen when the oxidation process is conducted on the (100) surface andwhen the oxidation process is conducted on the (111) surface, as long asthe Kr/O₂ plasma process is used for the oxidation process, as explainedalready. Further, it can be seen that substantially the same oxidationrate is achieved in the case of oxidizing the polysilicon film. Further,it should be noted that the oxidation rate thus obtained issubstantially identical with the oxidation rate observed when applying athermal oxidation process to a polysilicon film. In contrast, it can beseen that, when the conventional thermal oxidation process is applied,the oxidation rate of the Si substrate surface is much slower,indicating that the oxide film thus formed has a much smaller thickness.

From FIG. 12, it will be understood that a nearly identical oxidationrate is achieved for a Si surface as long as the Kr/O₂ plasma is usedfor the oxidation process, irrespective of whether the Si surface is asurface of a single-crystal Si of an arbitrary orientation or apolycrystalline surface including grain boundaries.

FIG. 13A shows the result of atomic-force microscopy applied to thesurface of a polysilicon film thus formed before the oxidation processis conducted.

FIG. 13B, on the other hand, shows the state of the polysilicon surfaceof FIG. 13A after the Kr/O₂ plasma processing is conducted. In the stateof FIG. 13B, it should be noted that the polysilicon surface is coveredby the oxide film formed as a result of the Kr/O₂ plasma process.Further, FIG. 13C shows the polysilicon surface in the state the oxidefilm is removed from the surface of FIG. 13B by an HF processing.

Referring to FIGS. 13A-13C, the oxidation process in the Kr/O₂ plasma iseffective at low temperature as low as 400° C., and there is caused nosubstantial grain growth in the polysilicon film. Associated therewith,there is no problem of surface roughening in the polysilicon film. Theoxide film thus has a generally uniform thickness.

In contrast, FIG. 14A shows the surface state of a polysilicon filmsubjected to thermal oxidation process at 900° C. in the state that thepolysilicon film carries thereon the oxide film, while FIG. 14B showsthe surface state in which the oxide film of FIG. 14A is removed.

Referring to FIGS. 14A and 14B, it can be seen that there occurs asubstantial crystal grain growth in the polysilicon film as a result ofthe thermal processing, and that there has been caused a substantialroughening in the polysilicon film surface. When a thin oxide film isformed on such a rough surface, there tends to occur the problem ofconcentration of electric field, while such a concentration of theelectric field causes the problem in the leakage current characteristicsand problems in the breakdown characteristics.

FIGS. 15A and 15B represent the result of transmission microscopicobservation showing the cross-section of the specimen in which an oxidefilm is formed on a polysilicon film surface by the Kr/O₂ plasmaprocessing. It should be noted that FIG. 15B shows a part of the area ofFIG. 15A in an enlarged scale.

Referring to FIG. 15A, it can be seen that there is formed an Al layeron the oxide film (designated as “polyoxide”), wherein FIG. 15A clearlyshows that the oxide film thus formed has a uniform thickness on thepolysilicon film surface. Further, the enlarged view of FIG. 15Bindicates that the oxide film is uniform.

FIG. 16 shows the relationship between the current density of thesilicon oxide film thus formed on the polysilicon film and the electricfield applied thereto, in comparison with a corresponding relationshipfor a thermal oxide film. Further, FIG. 17 is a diagram that shows therelationship of FIG. 16 in the Fowler-Nordheim plot.

Referring to FIGS. 16 and 17, it can be seen that the tunneling currentstarts to increase in the case the oxide film is formed on thepolysilicon film by the Kr/O₂ plasma oxidation process when the appliedelectric field has exceeded 5 MV/cm. Further, the plot of FIG. 17indicates that the tunneling current flowing through the oxide film is aFowler-Nordheim tunneling current, similarly to the case of the thermaloxide film. Further, from FIG. 17, it can be seen that there appears alarger barrier height φ_(B) of tunneling in the case the oxide film isformed by the Kr/O₂ plasma oxidation process as compared with the caseof the thermal oxide film. Further, it can be seen that there is causedan increase of breakdown voltage as compared with the case ofconventional thermal oxide film.

Fourth Embodiment

Next, the construction of a flash memory device according to a fourthembodiment of the present invention will be described with reference toFIG. 18, wherein the flash memory device of the present embodiment usesthe art of the low-temperature oxide film formation conducted in themicrowave plasma explained before.

Referring to FIG. 18, the flash memory device is constructed on asilicon substrate 1001 and includes a tunneling oxide film 1002 formedon the silicon substrate 1001 and a first polysilicon gate electrode1003 formed on the tunneling oxide film 1002 as a floating gateelectrode, wherein the polysilicon gate electrode 1003 is covered by asilicon oxide film 1004, and a second polysilicon gate electrode 1008 isformed on the silicon oxide film 1004 as a control gate electrode. InFIG. 18, illustration of source region, drain region, contact holes,interconnection patterns, and the like is omitted.

In the flash memory device of such a construction, a high quality filmcharacterized by small leakage current is obtained for the oxide film1004 as a result of the exposure of the polysilicon gate electrode 1003to the high-density plasma that is formed in the microwave plasmaprocessing apparatus of FIG. 2 by using the Kr/O₂ plasma gas. Thus, itbecomes possible to reduce the thickness of the oxide film 1004, andlow-voltage driving of the flash memory device becomes possible.

In the flash memory device of FIG. 18, it is also possible to use anitride film 1005 formed by the Kr/NH₃ plasma processing as explainedbefore in place of the oxide film 1004. Further, it is also possible touse an oxynitride film 1009 as explained before with reference to theprevious embodiment.

Fifth Embodiment

Next, the fabrication process of a flash memory device according to afifth embodiment of the present invention will be described, wherein theflash memory device of the present embodiment uses the technology oflow-temperature formation of oxide film and nitride film while using themicrowave plasma explained above, wherein the present embodiment alsoincludes a high-voltage transistor and a low-voltage transistor having agate electrode of polysilicon/silicide stacked structure.

FIG. 19 shows the schematic cross-sectional structure of a flash memorydevice 1000 according to the present embodiment.

Referring to FIG. 19, the flash memory device 1000 is constructed on thesilicon substrate 1001 and includes the tunneling oxide film 1002 formedon the silicon substrate 1001 and the first polysilicon gate electrode1003 formed on the tunneling oxide film 1002 as a floating gateelectrode, wherein the polysilicon gate electrode 1003 is furthercovered consecutively by the silicon nitride film 1004, a silicon oxidefilm 1005, a silicon nitride film 1006 and a silicon oxide film 1007,and the second polysilicon gate electrode 1008 is formed further on thesilicon nitride film 1007 as a control gate electrode. In FIG. 19,illustration of source region, drain region, contact holes,interconnection patterns, and the like, is omitted.

In the flash memory of the present embodiment, the silicon oxide films1002, 1005 and 1007 are formed according to the process of silicon oxidefilm formation explained before. Further, the silicon nitride films 1004and 1006 are formed according to the process of silicon nitride filmformation explained before. Thus, excellent electric property isguaranteed even when the thickness of these films is reduced to one-halfthe thickness of conventional oxide film or nitride film.

Next, the fabrication process of a semiconductor integrated circuitincluding the flash memory device of the present embodiment will beexplained with reference to FIGS. 20-25.

Referring to FIG. 20, a silicon substrate 101 carries a field oxide film1102 such that the field oxide film 1102 defines, on the siliconsubstrate 1101, a flash memory cell region A, a high-voltage transistorregion B and a low-voltage transistor region C, wherein each of theregions A-C is formed with a silicon oxide film 1103. The field oxidefilm 1102 may be formed by a selective oxidation (LOCOS) process or ashallow trench isolation process.

In the present embodiment, a Kr gas is used for the plasma excitationgas at the time of formation of the oxide film and the nitride film.Further, the microwave plasma processing apparatus of FIG. 2 is used forthe formation of the oxide film and the nitride film.

Next, in the step of FIG. 21, the silicon oxide film 1103 is removed inthe memory cell region A and a tunneling oxide film 1104 is formed onthe memory cell region A with a thickness of about 5 nm. During theformation of the tunneling oxide film 1104, the vacuum vessel (reactionchamber) 101 is evacuated to a vacuum state and the Kr gas and an O₂ gasis introduced from the shower plate 102 such that the pressure inside ofthe reaction chamber reaches 1 Torr (about 133 Pa). Further, thetemperature of the silicon wafer is set to 450° C., and a microwave of2.56 GHz frequency in the coaxial waveguide 105 is supplied to theinterior of the processing chamber via the radial line slot antenna 106and the dielectric plate 107. As a result, there is formed ahigh-density plasma.

In the step of FIG. 21, a first polysilicon film 1105 is deposited,after the step of forming the tunneling oxide film 1104, such that thefirst polysilicon film 1105 covers the tunneling oxide film 1104, andthe surface of the polysilicon film 1105 thus deposited is planarized byconducting a hydrogen radical processing. Further, the first polysiliconfilm 1105 is removed from the high-voltage transistor region B and thelow-voltage transistor region by way of patterning, leaving the firstpolysilicon film 1105 selectively on the tunneling oxide film 1104 ofthe memory cell region.

Next, in the step of FIG. 22, a lower nitride film 1106A, a lower oxidefilm 1106B, an upper nitride film 1106C and an upper oxide film 1106Dare formed consecutively on the structure of FIG. 21. As a result, aninsulation film 1106 having an NONO structure is formed by using themicrowave plasma processing apparatus of FIG. 2.

In more detail, the vacuum vessel (processing chamber) 101 of themicrowave plasma processing apparatus of FIG. 2 is evacuated to ahigh-vacuum state, and the Kr gas, an N₂ gas and an H₂ gas areintroduced into the processing chamber 101 from the shower plate 102until the pressure inside the processing chamber is set to about 100mTorr (about 13 Pa). Further, the temperature of the silicon wafer isset to 500° C. In this state, a microwave of 2.45 GHz frequency isintroduced into the processing chamber from the coaxial waveguide 105via the radial line slot antenna 106 and the dielectric plate 107, andthere is formed a high-density plasma in the processing chamber. As aresult of this, a silicon nitride film of about 6 nm thickness is formedon the polysilicon surface as the lower nitride film 1106A.

Next, the supply of the microwave is interrupted. Further, the supply ofthe Kr gas, the N₂ gas and the H₂ gas is interrupted, and the vacuumvessel (processing chamber) 101 is evacuated. Thereafter, the Kr gas andthe O₂ gas are introduced again into the processing chamber via theshower plate 102, and the pressure in the processing chamber is set to 1Torr (about 133 Pa). In this state, the microwave of 2.45 GHz frequencyis supplied again, and there is formed high-density plasma in theprocessing chamber 101. As a result, a silicon oxide film of about 2 nmthickness is formed as the lower oxide film 1106B.

Next, the supply of the microwave is again interrupted. Further, thesupply of the Kr gas and the O₂ gas is interrupted, and the processingchamber 101 is evacuated. Thereafter, the Kr gas, the N₂ gas and the H₂gas are introduced into the processing chamber via the shower plate 102so that the pressure inside the processing chamber is set to 100 mTorr(about 13 Pa). In this state, a microwave of 2.45 GHz frequency isintroduced and high-density plasma is formed in the processing chamber101. As a result of the plasma processing using the high-density plasmathus formed, there is further formed a silicon nitride film of 3 nmthickness.

Finally, the supply of the microwave is interrupted. Further, the supplyof the Kr gas, the N₂ gas and the H₂ gas is also interrupted, and thevacuum vessel (processing chamber) 101 is evacuated. Thereafter, the Krgas and the O₂ gas are introduced again via the shower plate 102 and thepressure inside the processing chamber is set to 1 Torr (about 133 Pa).In this state, the microwave of 2.45 GHz frequency is again supplied,and high-density plasma is formed in the processing chamber 101. As aresult, a silicon oxide film of 2 nm thickness is formed as the upperoxide film 1106D.

Thus, according to the foregoing process steps, it becomes possible toform the insulation film 1106 of the NONO structure with a thickness of9 nm. It was confirmed that the NONO film 1106 thus formed does notdepends on the surface orientation of polysilicon and that each of theoxide films and the nitride films therein is highly uniform in terms offilm thickness and film quality.

In the step of FIG. 22, the insulation film 1106 thus formed is furthersubjected to a patterning process such that the insulation film 1106 isselectively removed in the high-voltage transistor region B and in thelow-voltage transistor region C.

Next, in the step of FIG. 23, an ion implantation process is conductedinto the high-voltage transistor region B and further into thelow-voltage transistor region C for the purpose of threshold control.Thereafter, the oxide film 1103 is removed from the foregoing regions Band C, and a gate oxide film 1107 is formed on the high-voltagetransistor region B with a thickness of 7 nm, followed by the formationof a gate oxide film 1108 on the low-voltage transistor region C with athickness of 3.5 nm.

In the step of FIG. 23, the overall structure including the field oxidefilm 1102 is covered consecutively with a second polysilicon film 1109and a silicide film 1110. By patterning the polysilicon film 1109 andthe silicide film 1110, a gate electrode 111B is formed in thehigh-voltage transistor region B and a gate electrode 111C is formed inthe low-voltage transistor region C. Further, the polysilicon film 1109and the silicide film 110 are patterned in the memory cell region, and agate electrode 1111A is formed.

Finally, a standard semiconductor process including formation of sourceand drain regions, formation of insulation films, formation of contactholes and formation of interconnections, is conducted, and thesemiconductor device is completed.

It should be noted that the silicon oxide film and the silicon nitridefilm in the NONO film 1101 thus formed shows excellent electricproperties in spite of the fact that the each of the silicon oxide andsilicon nitride films therein has a very small thickness. Further, thesilicon oxide film and the silicon nitride film are dense and have afeature of high film quality. As the silicon oxide film and the siliconnitride film are formed at low temperature, there occurs no problem ofthermal budget formation, and the like, at the interface between thegate polysilicon and the oxide film, and an excellent interface isobtained.

In the flash memory integrated circuit device in which the flash memorydevices of the present invention are arranged in a two-dimensionalarray, it becomes possible to carry out writing and erasing ofinformation at low voltage. Further, the semiconductor integratedcircuit has advantageous features of suppressing substrate current andsuppressing degradation of the tunneling insulation film. Thus, thesemiconductor integrated circuit has a reliable device characteristic.The flash memory device of the present invention is characterized by alow leakage current, and enables writing of information at a voltage ofabout 7 V. Further, the flash memory device of the present invention canretain the written information over a duration longer than aconventional flash memory device by a factor of 10. The number of timesthe rewriting can be made is increased also by a factor of 10 in thecase of the flash memory of the present invention over a conventionalflash memory device.

Sixth Embodiment

Next, a flash memory device according to a second embodiment of thepresent invention will be described, wherein the flash memory device ofthe present embodiment has a gate electrode having apolysilicon/silicide stacked structure and is formed by using the art oflow-temperature formation of oxide and nitride film that uses thehigh-density microwave plasma explained before.

FIG. 24 shows a schematic cross-sectional structure of a flash memorydevice 1500 according to the present embodiment.

Referring to FIG. 24, the flash memory device 1500 is constructed on asilicon substrate 1501 and includes a tunneling nitride film 1502 formedon the silicon substrate 1501 and a first polysilicon gate electrode1503 formed on the tunneling nitride film 1502 as a floating gateelectrode, wherein the first polysilicon gate electrode 1503 is coveredconsecutively by a silicon oxide film 1504, a silicon nitride film 1505and a silicon oxide film 1506. Further, a second polysilicon electrode1507 forming a control gate electrode is formed on the silicon oxidefilm 1506. In FIG. 24, illustration of source region, drain region,contact holes, interconnection patterns, and the like, is omitted.

In the flash memory device 1500 of FIG. 24, the silicon oxide films1502, 1504 and 1506 are formed according to a process of forming asilicon oxide film that uses the high-density microwave plasma explainedbefore. Further, the silicon nitride film 1505 is formed by a process offorming a silicon nitride film that uses the high-density microwaveplasma explained before.

In the present embodiment, too, the process steps up to the step ofpatterning the first polysilicon film 1503 are identical with those ofthe steps of FIGS. 20 and 21, except for the point that the tunnelingnitride film 1502 is formed after the step of evacuating the vacuumvessel (processing chamber) 101, by introducing an Ar gas, an N₂ gas andis an H₂ gas from the shower plate 102 such that the pressure inside theprocessing chamber becomes 100 mTorr (about 13 Pa). Thereby, thetunneling nitride film 1502 is formed to have a thickness of about 4 nm,by supplying a microwave of 2.45 GHz to form high-density plasma in theprocessing chamber.

After the first polysilicon film 1503 is thus formed, the lower siliconoxide film 1504 and the silicon nitride film 1505 and the upper siliconoxide film 1506 are formed consecutively on the first polysilicon film,and an insulation film having an ONO structure is obtained.

In more detail, the vacuum chamber (processing chamber) 101 of themicrowave plasma processing apparatus explained previously withreference to FIG. 2 is evacuated to a high vacuum state, and the Kr gasand an O₂ gas are introduced into the processing chamber via the showerplate 102 such that the pressure of the processing chamber 101 is set to1 Torr (about 133 Pa). In this state, the microwave of 2.45 GHz issupplied to the processing chamber 101 and there is formed thehigh-density plasma therein. As a result, a silicon oxide film having athickness of about 2 nm is formed on the surface of the firstpolysilicon film 1503.

Next, a silicon nitride film is formed on the silicon oxide film by aCVD process with a thickness of 3 nm, and the vacuum vessel (processingchamber) 101 is evacuated. Further, the Ar gas, the N₂ gas and the H₂gas are introduced into the processing chamber via the shower plate 102,and the pressure inside the processing chamber is set to 1 Torr (about133 Pa). In this state, the microwave of 2.45 GHz is supplied again andthe high-density plasma is formed in the processing chamber 101. Byexposing the foregoing silicon nitride film to the hydrogen nitrideradicals NH* formed with the high-density plasma, the silicon nitridefilm is converted to a dense silicon nitride film.

Next, a silicon oxide film is formed on the foregoing dense siliconnitride film by a CVD process with a thickness of about 2 nm, and thepressure of the processing chamber 101 of the microwave plasmaprocessing apparatus is set to 1 Torr (about 133 Pa) by supplyingthereto the Kr gas and the O₂ gas. By supplying the microwave of 2.45GHz further to the processing chamber in this state, the high-densityplasma is formed in the processing chamber 101. Thereby, the CVD oxidefilm formed previously in the CVD process is converted to a densesilicon oxide film by exposing to the atomic state oxygen O* formed withthe high-density plasma.

Thus, an ONO film is formed on the polysilicon film 1503 with athickness of about 7 nm. The ONO film thus formed shows no dependence ofproperty thereon on the orientation of the polysilicon surface on whichthe ONO film is formed and has an extremely uniform thickness. The ONOfilm thus formed is then subjected to a patterning process for removinga part thereof corresponding to the high-voltage transistor region B andthe low-voltage transistor region C. By further applying the processsteps similar to those used in the fourth embodiment before, the devicefabrication process is completed.

The flash memory device thus formed has an excellent leakagecharacteristic characterized by low leakage current, and writing andreading operation can be conducted at the voltage of about 6V. Further,the flash memory device provides a memory retention time larger by thefactor of 10 over the conventional flash memory devices, similarly tothe flash memory device 1000 of the previous embodiment. Further, it ispossible to achieve the number of rewriting operations larger by thefactor of 10 over the conventional flash memory devices.

Seventh Embodiment

Next, a description will be made on a flash memory device 1600 accordingto a seventh embodiment of the present invention, wherein the flashmemory device 1600 has a gate electrode of polysilicon/silicide stackedstructure and is formed by the process that uses the microwavehigh-density plasma for forming low temperature oxide and nitride films.

FIG. 25 shows the schematic cross-sectional structure of the flashmemory device 1600.

Referring to FIG. 25, the flash memory device 1600 is constructed on asilicon substrate 1601 and includes a tunneling oxide film 1602 formedon the silicon substrate 1061 and a first polysilicon gate electrode1603 formed on the tunneling oxide film 1602, wherein the firstpolysilicon gate electrode 1603 is covered consecutively by a siliconnitride film 1604 and a silicon oxide film 1605.

Further, a second polysilicon gate electrode 1606 is formed on thesilicon oxide film 1605 as a control gate electrode.

In FIG. 25, illustration of source region, drain region, contact holes,and interconnection patterns, is omitted.

In the flash memory 1600 of FIG. 25, the silicon oxide films 1602 and1605 are formed by the film forming process of oxide film explainedabove, while the silicon nitride film 1604 is formed by the film formingprocess of nitride film also explained above.

Next, the fabrication process of a flash memory integrated circuitaccording to the present invention will be explained.

In the present embodiment, too, the process proceeds similarly to theprevious embodiments up to the step of patterning the first polysiliconfilm 1603, and the first polysilicon film 1603 is formed in the regionA. Thereafter, an insulation film having an NO structure is formed byconsecutively depositing a silicon nitride film and a silic0on oxidefilm on the first polysilicon film 1603.

In more detail, the NO film is formed by using the microwave plasmaprocessing apparatus of FIG. 2 according to the process steps notedbelow.

First, the vacuum vessel (processing chamber) 101 is evacuated, and a Krgas, an N₂ gas and an H₂ gas are introduced thereto via the shower plate102 and the pressure inside the processing chamber is set to about 100mTorr (about 13 Pa). In this state, a microwave of 2.45 GHz is supplied,and high-density plasma is induced in the processing chamber. Thereby,there occurs a nitriding reaction in the polysilicon film 1603 and asilicon nitride film is formed with a thickness of about 3 nm.

Next, a silicon oxide film is formed by a CVD process to a thickness ofabout 2 nm, and a Kr gas and an O₂ gas are introduced in the microwaveplasma processing apparatus such that the pressure inside the processingchamber is set to about 1 Torr (about 133 Pa). In this sate, a microwaveof 2.45 GHz frequency is supplied to form high-density plasma in theprocessing chamber, such that the oxide film formed by the CVD processis exposed to the atomic state oxygen O* associated with thehigh-density plasma. As a result, the CVD oxide film is converted to adense silicon oxide film.

The NO film is thus formed to a thickness of about 5 nm, wherein the NOfilm thus formed has an extremely uniform thickness irrespective of thesurface orientation of the polysilicon crystals. The NO film thus formedis then subjected to a patterning process and the part thereof coveringthe high-voltage transistor region B and the low-voltage transistorregion C are removed selectively.

After the foregoing process, the process steps similar to those of FIG.23 are conducted and the device fabrication process is completed.

It should be noted that the flash memory device thus formed has a lowleakage characteristic, and enables writing or erasing at a low voltageas low as 5 V. Further, the flash memory device provides a memoryretention time larger than the conventional memory retention time by afactor of 10, and rewriting cycles larger than the conventionalrewriting cycles by a factor of 10.

It should be noted that the fabrication process of the memory cell, thehigh-voltage transistor and the low-voltage transistor merely representsan example, and the present invention is by no means limited to theseembodiments. For example, it is possible to use an Ar gas in place ofthe Kr gas during the formation process of the nitride film.

Further, it is possible to use a film having a stacked structure ofpolysilicon/silicide/polysilicon/refractory metal/amorphous silicon orpolysilicon, for the first and second polysilicon films.

Further, it is also possible to use another plasma processing apparatusin place of the microwave plasma processing apparatus of FIG. 2 forforming the oxide film or nitride film of the present invention, as longas the plasma processing apparatus enables low temperature formation ofan oxide film. Further, the radial line slot antenna is not the onlysolution for introducing a microwave into the processing chamber of theplasma processing apparatus, and the microwave may be introduced byother means.

In place of the microwave plasma processing apparatus of FIG. 2, it isalso possible to use a plasma processing apparatus having a two-stageshower plate construction, in which the plasma gas such as the Kr gas orAr gas is introduced from a first shower plate and the processing gas isintroduced from a second shower plate different from the first showerplate. In this case, it is also possible to introduce the oxygen gasfrom the second shower plate. Further, it is possible to design theprocess such that the floating gate electrode of the flash memory deviceand the gate electrode of the high-voltage transistor are formedsimultaneously by the first polysilicon electrode.

Further, the present invention is not limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

INDUSTRIAL APPLICABILITY

According to the present invention, it becomes possible to form ahigh-quality silicon oxide film, silicon nitride film or siliconoxynitride film on a polysilicon film with excellent characteristics andreliability comparable with, or superior to, those of a silicon thermaloxide film formed at a high temperature of about 1000° C. or a CVDsilicon nitride film, by using a Kr-containing insulation film formed bya novel plasma oxidation process or nitridation process conducted at alow temperature lower than 550° C. Thus, the present invention realizesa high quality and high-performance flash memory device, which allowsrewriting operation at low voltage and provides excellent electriccharge retention characteristic.

1. A method of forming a silicon oxynitride film on a polysilicon film,characterized by the steps of: forming plasma containing therein atomicstate oxygen O* and hydrogen nitride radicals NH* in a processing vesselof a microwave processing apparatus, said microwave processing apparatusincluding, in addition to said processing vessel, a shower plateprovided in part of said processing vessel so as to extend parallel witha substrate to be processed, said shower place including a number ofapertures for supplying a plasma gas toward said substrate to beprocessed, and a microwave radiation antenna provided such that saidmicrowave radiation antenna emits a microwave into said processingvessel through said shower plate, said plasma being formed by supplyingan inert gas predominantly of Ar or Kr and a gas containing oxygen as aconstituent element and a gas containing nitrogen as a constituentelement into said processing vessel via said shower plate, and bysupplying a microwave into said processing vessel from said microwaveradiation antenna through said shower plate; and oxynitriding, in saidprocessing vessel, a surface of said polysilicon film formed on saidsubstrate by said plasma, to form said silicon oxynitride film.
 2. Amethod of forming a silicon oxynitride film as claimed in claim 1,characterized in that said gas containing nitrogen and hydrogen is anNH₃ gas.
 3. A method of forming a silicon oxynitride film as claimed inclaim 1, characterized in that said gas containing nitrogen and hydrogenis a mixed gas of an N₂ gas and an H₂ gas.
 4. A method of forming asilicon oxynitride film as claimed in claim 1, characterized in thatsaid plasma has an electron density of 10¹²cm⁻³or more at said surfaceof said polysilicon film.